Differences between GNU Emacs and CCA Emacs.
Copyright (c) 1985 Richard M. Stallman

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* GNU Emacs Lisp vs CCA Elisp.

GNU Emacs Lisp does not have a distinction between Lisp functions
and Emacs functions, or between Lisp variables and Emacs variables.
The Lisp and the editor are integrated.  A Lisp function defined
with defun is callable as an editor command if you put an
interactive calling spec in it; for example, 
  (defun forward-character (n)
    (interactive "p")
    (goto-char (+ (point) n)))
defines a function of one argument that moves point forward by
a specified number of characters.  Programs could call this function,
as in (forward-character 6), or it could be assigned to a key,
in which case the "p" says to pass the prefix numeric arg as
the function's argument.  As a result of this feature, you often
need not have two different functions, one to be called by programs
and another to read arguments from the user conveniently; the same
function can do both.

CCA Elisp tries to be a subset of Common Lisp and tries to
have as many Common Lisp functions as possible (though it is still
only a small fraction of full Common Lisp).  GNU Emacs Lisp
is somewhat similar to Common Lisp just because of my Maclisp
and Lisp Machine background, but it has several distinct incompatibilities
in both syntax and semantics.  Also, I have not attempted to
provide many Common Lisp functions that you could write in Lisp,
or others that provide no new capability in the circumstances.

GNU Emacs Lisp does not have packages, readtables, or character objects
(it uses integers to represent characters).

On the other hand, windows, buffers, relocatable markers and processes
are first class objects in GNU Emacs Lisp.  You can get information about them
and do things to them in a Lispy fashion.  Not so in CCA Emacs.

In GNU Emacs Lisp, you cannot open a file and read or write characters
or Lisp objects from it.  This feature is painful to support, and
is not fundamentally necessary in an Emacs, because instead you
can read the file into a buffer, read or write characters or
Lisp objects in the buffer, and then write the buffer into the file.

On the other hand, GNU Emacs Lisp does allow you to rename, delete, add
names to, and copy files; also to find out whether a file is a
directory, whether it is a symbolic link and to what name, whether
you can read it or write it, find out its directory component,
expand a relative pathname, find completions of a file name, etc.,
which you cannot do in CCA Elisp.

GNU Emacs Lisp uses dynamic scope exclusively.  This enables you to
bind variables which affect the execution of the editor, such as
indent-tabs-mode.

GNU Emacs Lisp code is normally compiled into byte code.  Most of the
standard editing commands are written in Lisp, and many are
dumped, pure, in the Emacs that users normally run.

GNU Emacs allows you to interrupt a runaway Lisp program with
Control-g.

* GNU Emacs Editing Advantages

GNU Emacs is faster for many things, especially insertion of text
and file I/O.

GNU Emacs allows you to undo more than just the last command
with the undo command (C-x u, or C-_).  You can undo quite a ways back.
Undo information is separate for each buffer; changes in one buffer
do not affect your ability to undo in another buffer.

GNU Emacs commands that want to display some output do so by putting
it in a buffer and displaying that buffer in a window.  This
technique comes from Gosling Emacs.  It has both advantages and
disadvantages when compared with the technique, copied by CCA Emacs
from my original Emacs which inherited it from TECO, of having "type
out" which appears on top of the text in the current window but
disappears automatically at the next input character.

GNU Emacs does not use the concept of "subsystems".  Instead, it uses
highly specialized major modes.  For example, dired in GNU Emacs has
the same commands as dired does in other versions of Emacs, give or
take a few, but it is a major mode, not a subsystem.  The advantage
of this is that you do not have to "exit" from dired and lose the
state of dired in order to edit files again.  You can simply switch
to another buffer, and switch back to the dired buffer later.  You
can also have several dired buffers, looking at different directories.

It is still possible to write a subsystem--your own command loop--
in GNU Emacs, but it is not recommended, since writing a major mode
for a special buffer is better.

Recursive edits are also rarely used, for the same reason: it is better
to make a new buffer and put it in a special major mode.  Sending
mail is done this way.

GNU Emacs expects everyone to use find-file (C-x C-f) for reading
in files; its C-x C-v command kills the current buffer and then finds
the specified file.

As a result, users do not need to think about the complexities
of subsystems, recursive edits, and various ways to read in files
or what to do if a buffer contains changes to some other file.

GNU Emacs uses its own format of tag table, made by the "etags"
program.  This format makes finding a tag much faster.

Dissociated Press is supported.


* GNU Emacs Editing Disadvantages.

GNU Emacs does not display the location of the mark.

GNU Emacs does not have a concept of numbers of buffers,
or a permanent ordering of buffers, or searching through multiple
buffers.  The tags-search command provides a way to search
through several buffers automatically.

GNU Emacs does not provide commands to visit files without
setting the buffer's default directory.  Users can write such
commands in Lisp by copying the code of the standard file
visiting commands and modifying them.

GNU Emacs does not support "plus options" in the command
arguments or in buffer-selection commands, except for line numbers.

GNU Emacs does not support encryption.  Down with security!

GNU Emacs does not support replaying keystroke files,
and does not normally write keystroke files.

GNU Emacs does not support the Life game.


* Neutral Differences

GNU Emacs uses TAB, not ESC, to complete file names, buffer names,
command names, etc.

GNU Emacs uses ESC to terminate searches, instead of
the C-d uses by CCA Emacs.  (Actually, this character is controlled
by a parameter in GNU Emacs.)  C-M-s in GNU Emacs is an interactive
regular expression search, but you can get to a noninteractive
one by typing ESC right after the C-M-s.  GNU Emacs searches
never wrap around at beginning or end of buffer.

In GNU Emacs, C-x s asks, for each modified file buffer, whether
to save it.

GNU Emacs indicates line continuation with "\" and line
truncation (at either margin) with "$".

The command to resume a tags-search or tags-query-replace in
GNU Emacs is Meta-Comma.
